MXenes and their composites for advanced cathodes in multivalent ion batteries

Physical properties of MXenes

An outstanding advantage of MXenes and their derivatives is their high electrical conductivity, which plays a vital role in enhancing ion and electron transfer. The Ti₃C₂T_x film was reported to exhibit an impressive electrical conductivity value of approximately 9,880 S cm^-1 with a thin film thickness of 88 nm^[14]. In addition, the 2D structure of MXenes has a high specific surface area and aspect ratio. This unique structure allows for better contact between the electrolyte and the electrode material, creating more active sites for electrochemical reactions. The nitrogen-doping Ti₃C₂T_x has been reported to have an open pore structure and a high specific surface area of up to 368.8 m² g^-1[15], while its MAX phase is only 5.1 m² g^-1.

Additionally, MXenes exhibit excellent strength and toughness. They can withstand high external loads and stresses without breaking or deforming. MXenes also exhibit remarkable hardness, which allows them to withstand scratches and abrasions caused by external forces. A 5-mm-thick hollow Ti₃C₂T_x cylinder can withstand 4,000 times its own weight^[16]. Flexible Ti₃C₂T_x MXene paper has been reported to have a tensile strength of 83.2 MPa and a conductivity of 265,600 S m^-1 and can be folded into small windmills^[30]. These characteristics make MXenes an excellent electrode choice for flexible batteries and wearable electronics.

Chemical properties of MXenes

The remarkable chemical stability of MXenes makes them ideal for energy-storage devices, maintaining stability even in complex electrolyte environments^[18]. This extends the lifespan of the devices and enhances their reliability and durability. Additionally, their negatively charged surface can be easily combined with other positively charged materials through an electrostatic self-assembly strategy^[27], and the layered structure of MXenes enables electrodes to expose more active sites for charge storage, further enhancing their potential for energy-storage applications^[31]. A heterostructure material (Cu-HHTP/MX) composed of Cu-HHTP (HHTP = 2,3,6,7,10,11-hexahydroxytriphenylene) and V₂CT_x MXene was prepared via electrostatic self-assembly^[31]. Brunauer-Emmett-Teller (BET) results show that Cu-HHTP/MX has a larger surface area (152.6 m² g^-1) than Cu-HHTP (90.88 m² g^-1), suggesting that MXene can increase the number of active sites for the insertion/extraction of Zn²⁺ ions.

MXene materials can be tailored to meet specific performance requirements by adjusting their elemental compositions and terminal functional groups (T_x), such as -O, -OH, and -F. According to the reports^[32,33], owing to fluorine-containing acid etching, the surface terminations of MXenes inevitably generate abundant -F, which hinders Zn²⁺ ion transfer in ZIBs. To solve this problem, OH-termination-rich V₂CT_x material was prepared through a one-step alkalization method^[34]. The tailored MXene realized fast Zn²⁺ ion transfer, exhibiting outstanding electrochemical performance (498.2 mA h g^-1 at 0.1 A g^-1, and 195.1 mA h g^-1 at 30 A g^-1) and excellent cycle stability (96.2% capacity retention upon 20,000 cycles).

"Top-down" selective etching method

The “top-down” method removes the A-layer atoms in the MAX phase through direct or indirect chemical etching and then peels them to form single- or multilayer MXene nanosheets. Most MXenes are currently obtained using the “top-down” method, and the typical synthesis method is hydrofluoric acid (HF) etching. Ti₃C₂T_x was first synthesized using a HF solution to etch the Al layer of the Ti₃AlC₂ MAX phase, as shown in Figure 3A^[35]. During the synthesis of MXenes, some crucial parameters exist, such as the HF concentration, etching time, and reaction temperature^[36]. However, the strong corrosiveness and toxicity of HF render the preparation process dangerous. Other fluorine-based etchers have also been investigated. A mixture of hydrochloric acid (HCl) and lithium fluoride (LiF) as an alternative to HF was reported to synthesize Ti₃C₂T_x successfully [Figure 3B]^[37]. The resulting MXene is a clay-like product that can be used to prepare various electrodes and films. Since then, other fluorinated salts (NaF^[38,39], KF^[39], NH₄F^[39,40], and FeF₃^[41]) have proven useful for the synthesis of MXenes. In addition to aqueous solution etching, MXenes have also been synthesized using the high-temperature molten salt method. The specific approach is to heat the salt mixture (29 wt% LiF, 59 wt% KF, and 12 wt% NaF) and Ti₄AlN₃ at 550 °C for 30 min under an argon (Ar) atmosphere to produce Ti₄N₃T_x [Figure 3C]^[42]. Organic solvents have also been used for water-free etching of MXenes. According to the literature^[43], NH₄HF₂ can be dissociated into HF and NH₄F in polar organic solvents (such as propylene carbonate [PC], N-methyl-2-pyrrolidone [NMP], and dioxane [DXN]). Therefore, Ti₃AlC₂ was etched using organic polar solvents in the presence of NH₄HF₂ to obtain Ti₃C₂T_z MXene, which expanded the use of MXene in water-sensitive applications [Figure 3D]. The transmission electron microscope (TEM) image in Figure 3E shows a typical Ti₃C₂T_z flake morphology, indicating the successful preparation of MXene.

Figure 3. (A) Schematic illustration of the synthesis of Ti₃C₂T_x by HF. This figure is quoted with permission from Naguib et al. Copyright (2011) John Wiley and Sons^[35]. (B) Schematic illustration of the preparation of Ti₃C₂T_x by HCl + LiF. This figure is quoted with permission from Ghidiu et al. Copyright (2014) Springer Nature^[37]. (C) Schematic illustration of the preparation of Ti₄N₃T_x by high-temperature molten salt method. This figure is quoted with permission from Urbankowski et al. Copyright (2016) RSC Pub^[42]. (D) Schematic illustration of the synthesis of Ti₃C₂T_x by organic polar solvents. (E) TEM image of delaminated Ti₃C₂T_x sheets etched by organic polar solvents. This figure is quoted with permission from Natu et al. Copyright (2020) Elsevier^[43]. (F) Schematic illustration of the preparation of Ti₃C₂T_x by alkali etching. This figure is quoted with permission from Li et al. Copyright (2018) John Wiley and Sons^[45]. (G) Schematic illustration of the synthesis of Ti₃C₂T_x by Lewis acidic molten melts etching. This figure is quoted with permission from Li et al. Copyright (2019) American Chemical Society^[46]. (H) Schematic illustration of the preparation of Ti₃C₂T_x by halogen etching. This figure is quoted with permission from Jawaid et al. Copyright (2021) American Chemical Society^[47].

Although the application of fluorine-based etchers (such as HCl + LiF, salt mixture (LiF + KF + NaF), and NH₄HF₂) reduces operational risk, it inevitably leads to environmental pollution. Consequently, researchers have redirected their focused on the synthesis of fluorine-free MXenes. Electrochemical etching has been demonstrated as an effective approach. Ti₃C₂T_x was obtained by anodic corrosion^[44]. The Al atoms on the anode side of the MAX phase were extracted and subsequently replaced with hydroxide groups in the NH₄Cl and TMAOH electrolytes. Notably, electrochemical etching utilizes a two-electrode system and no etching reactions occur at the cathode. Alkali etching has also been found to be useful for the synthesis of MXenes. It has been reported that Ti₃C₂T_x MXene can be obtained using NaOH (27.5 M) at 270 °C to etch Ti₃AlC₂, and the purity of Ti₃C₂T_x is as high as 92% [Figure 3F]^[45]. Furthermore, the utilization of Lewis acid to remove Zn atoms from the MAX phase is an effective method for synthesizing MXenes. It requires the transformation from the MAX phase to a Zn-based MAX phase at 550 °C in an Ar atmosphere via the replacement reaction [Figure 3G]^[46]. The resulting Ti₃ZnC₂ reacts with ZnCl₂ to form a chlorine (Cl)-terminated MXene. Considering the high-temperature conditions of Lewis acid, milder halogen etching has been proposed. For example, Ti₃AlC₂ was etched using halogens (Br₂, I₂, ICl, and IBr) in anhydrous media [Figure 3H]^[47].However, this type of etching can be affected by various factors such as the absolute concentration of the halogen, temperature, the solvent, and the molar ratio of the halogen to the MAX phase.

"Bottom-up" direct synthesis method

In the “bottom-up” method, MXenes are directly synthesized without etching the MAX phase. Synthesized MXenes have the advantages of a large size and few defects^[48]. Chemical vapor deposition (CVD) was the first direct synthesis method for MXenes. Mo₂C MXene was synthesized by reacting Mo atoms from Mo foil with carbon atoms produced from methane at a temperature above 1,085 °C^[49]. Mo₂C is a chemically stable crystal with a size exceeding 100 µm and a thickness of several nanometers. In addition to conventional CVD systems, large-area Mo₂C MXene has been grown using plasma-enhanced pulsed laser deposition^[50]. This method combines the advantages of both plasma-enhanced CVD and pulsed laser deposition. The temperature required for this crystal growth is 700 °C, and the laser pulse number can control crystal thickness. In addition, MXenes can be directly synthesized using the salt-templated method. MoO₃-coated NaCl was first synthesized using an Mo precursor and NaCl under an Ar atmosphere at 280 °C, and then MoO₃-coated NaCl was ammoniated under an NH₃ atmosphere at 650 °C to generate MoN MXene^[51]. However, large-scale application of MXenes is hindered by the low output, high equipment requirements, and high cost of the “bottom-up” method^[48]. Therefore, it is important to explore more efficient methods for expanding the applications of MXenes.

MXenes and their derivatives as cathodes in ZIBs

ZIBs have attracted considerable interest because of their eco-friendliness, cost-effectiveness, and safety^[54-57]. The Zn anode has a high theoretical specific capacity (820 mA h g^-1 and 5,851 mA h cm^-3), a low redox potential [-0.763 V vs. Standard Hydrogen Electrode (SHE)], and abundant reserves, making it an ideal large-scale energy-storage device^[58,59]. In the past few years, 2D MXenes have been extensively researched in ZIBs because of their high metal conductivity (close to graphene: 10⁵-10⁶ S m^-1), large layer spacing, structural stability, controllable surface functional groups, etc.

Although MXenes have many properties required for ZIB cathodes, the large amount of F surface terminations produced in the traditional fluoric acid etching process usually leads to high hydrophobicity and slow ion diffusion kinetics, which deteriorates the electrochemical performance. Therefore, the replacement of -F functional groups with functional groups that are hydrophilic (-OH) or electrochemically active (-I, -Br) is expected to improve the performance of MXenes. A novel OH-rich terminal V₂CT_x material with interlayer “K⁺-pillars” (alk-V₂CT_x) was proposed [Figure 5A]^[34]. According to the X-ray diffraction (XRD) patterns [Figure 5B], the (002) peak of alk-V₂CT_x shifted to a smaller angle, indicating that the inserted K⁺ ions increased the layer spacing^[60]. Simultaneously, the inserted K⁺ ions improved the structural stability of the alk-V₂CT_x cathode. Owing to the large number of hydrophilic -OH terminations, the hydrophilicity of alk-V₂CT_x was significantly better than that of traditional HF-etched V₂CT_x [Figure 5C], which is conducive to electrolyte infiltration in the electrode. The alk-V₂CT_x cathode had a specific capacity of 498.2 mA h g^-1 at 0.1 A g^-1, with a capacity retention of 96.2% after 20,000 cycles at 10 A g^-1.

Figure 5. (A) Illustration of hydrophilicity and structural stability’s improvement of alk-V₂CT_x. (B) XRD patterns of V₂AlC, V₂CT_x, and alk-V₂CT_x. (C) Comparison of hydrophilicity between V₂CT_x and alk-V₂CT_x. This figure is quoted with permission from Chen et al. Copyright (2023) John Wiley and Sons^[34]. (D) SEM image of Ti₃C₂Br₂. (E) HAADF-STEM image and EDS mappings of Ti₃C₂Br₂. (F) Charge and discharge curves of halogenated Ti₃C₂ MXenes at 0.5 A g^-1. This figure is quoted with permission from Li et al. Copyright (2021) American Chemical Society^[61]. (G) Schematic illustration of V₂CT_x cathodes at different voltages by in-situ electrochemical activation. (H and I) SEM images of V₂CT_x cathodes after electrochemical activation at 1.8 V and 2.0 V. This figure is quoted with permission from Liu et al. Copyright (2020) John Wiley and Sons^[62].

In addition to -OH terminations, various halogen terminals (such as -Cl and -Br) can improve the electrochemical properties of MXene. Ti₃C₂ MXenes with different halogen terminals were obtained by etching with different copper halide molten salts at high temperatures^[61]. After etching the Al layers, the scanning electron microscopy (SEM) image of Ti₃C₂Br₂ [Figure 5D] exhibited a multilayer structure similar to that of Ti₃C₂(OF) MXene (obtained via traditional HF etching), indicating successful halogenated MXene etching. The high-angle annular dark-field scanning transmission electron microscopy (HAADF-STEM) image and energy-dispersive spectroscopy (EDS) elemental mapping [Figure 5E] showed that bromine (Br) was uniformly distributed on Ti₃C₂Br₂. According to the galvanostatic charge and discharge (GCD) curves of Ti₃C₂ MXenes with different halogen terminals [Figure 5F], Ti₃C₂Br₂ had a specific capacity of 97.6 mA h g^-1 and a discharge platform of 1.6 V, whereas Ti₃C₂I₂ had a specific capacity of 135 mA h g^-1 and a discharge platform of 1.1 V. However, Ti₃C₂Cl₂ and Ti₃C₂(OF) both exhibited low capacity (51.7 and 46.5 mA h g^-1, respectively). The electron transfer between Br^- (I^-) and Br⁰ (I⁰) provides voltage platforms and higher capacity than Ti₃C₂Cl₂ and Ti₃C₂(OF), indicating that the surface chemical adjustment of MXenes is important for improving their electrochemical performance.

Compared with other MXenes, V₂C MXenes uniquely benefit from the multiple valence states of V and the high specific capacity of V-based cathodes. The oxidation of low-valence V in V₂C to obtain MXene-based derivatives is a unique method for enhancing the electrochemical properties of V₂C MXenes. The low-valence V in the V₂CT_x cathode was oxidized from V²⁺/V³⁺ to V⁴⁺/V⁵⁺ through initial charging activation, forming a nanoscale V oxide (VO_x) coating and achieving more Zn-ion storage^[62]. Currently, electrochemical activation, hydrothermal oxidation, and selenization are used to prepare MXene-based derivatives. V₂CT_x MXene cathodes have been held at different voltages for 2 h, and the obtained products were denoted as 1.4-V₂CT_x, 1.8-V₂CT_x, and 2.0-V₂CT_x [Figure 5G]. Among these MXenes, the outer surface of 1.8-V₂CT_x was coated with homogeneous nanoscale VO_x and the internal V-C-V multilayer structure was retained, whereas 2.0-V₂CT_x was over-oxidized [Figure 5H and I]. Activated high-valence V species provide additional capacity, and internally retained V-C-V layers ensure high conductivity of the cathodes. Thus, the 1.8-V₂CT_x cathode had a specific capacity of 358 mA h g^-1 even at 30 A g^-1. In contrast to traditional acid and alkali etching, an in-situ electrochemical method that removes the Al layer and enhances the V valence of V₂AlC has been proposed^[63]. During the electrochemical process, the V₂AlC cathode undergoes three sequential stages [Figure 6A]. (1) The Al layer is exfoliated by the fluorine-rich electrolyte; (2) V is gradually oxidized to V₂O₅; and (3) After a long cycle, the V₂O₅ particles are uniformly distributed on the retained carbon layer, leading to the formation of a V₂O₅/C/V₂O₅ structure. The resulting V₂O₅/C/V₂O₅ sandwich structure is shown in Figure 6B. The XRD patterns [Figure 6C] of the V₂AlC cathode after long-term cycling also demonstrate the presence of V₂O₅ particles and carbon layers. The obtained cathode had a capacity of 409.7 mA h g^-1 at 0.5 A g^-1 and 95.7 mA h g^-1 at 64 A g^-1.

Figure 6. (A) Schematic diagram of electrochemical activation of V₂AlC cathode at 5 A g^-1. (B) Schematic diagram of the resulting uniform distribution of V₂O₅ particles on two-dimensional carbon layers. (C) XRD patterns of V₂AlC cathode after long cycling at 5 A g^-1. This figure is quoted with permission from Li et al. Copyright (2020) John Wiley and Sons^[63]. (D) Schematic illustration of ion intercalation and oxidation process during MVO@VC preparation. (E) SEM image of K-V₂CT_x with the element mapping. (F) SEM image of MVO@VC with the element mapping. This figure is quoted with permission from Zhu et al. Copyright (2021) Elsevier^[66]. (G) Schematic representation of the synthetic process for VSe₂@V₂CT_x nanohybrids. This figure is quoted with permission from Sha et al. Copyright (2022) American Chemical Society^[67].

The electrochemical properties of cathodes can also be effectively enhanced through ion insertion. MXene-derived materials intercalated with metal ions (such as Mg^2+[64] and Zn^2+[65]) have also been extensively studied. Based on the significant ion-adsorption capacity of MXenes, V₂CT_x was alkalized with KOH (K-V₂CT_x). The Mn_xV₁₀O₂₄·nH₂O@V₂CT_x heterostructure (MVO@VC) was then synthesized from K-V₂CT_x by hydrothermal Mn²⁺ intercalation and oxidation [Figure 6D]^[66]. Through the hydrothermal process, K-V₂CT_x transformed from its original layered structure to a three-dimensional (3D) porous network structure [Figure 6E and F]. The MVO@VC cathode has a capacity of 287.6 mA h g^-1 at 10 A g^-1 and a retained 92.9% initial capacity after 25,000 cycles. Generally, the induction of V₂C MXene using electrochemical methods to obtain MXene-based derived materials is time-consuming and uncontrollable; thus, a simple surface selenization strategy has been proposed^[67]. As shown in Figure 6G, transition metal selenides were obtained from the surface layer of MXenes using selenium powder as an oxidant in an Ar atmosphere, whereas the inner layers of MXene remained. In this study, VSe₂@V₂CT_x nanohybrid was successfully synthesized. In addition to V₂CT_x, the successful selenization of Ti₃C₂T_x (TiSe₂@Ti₃C₂T_x) and Nb₂CT_x(NbSe₂@Nb₂CT_x) suggests that this approach is suitable for a broad range of MXenes. On the one hand, the presence of V₂CT_x facilitates ion and electron diffusion; on the other hand, VSe₂, with a stable structure, increases the capacity. The VSe₂@V₂CT_x cathode had a capacity of 231.3 mA h g^-1 after 100 cycles at 0.5 A g^-1 and 158.1 mA h g^-1 after 600 cycles at 2.0 A g^-1.

MXenes and their derivatives as cathodes in AIBs

Al metal reserves are abundant. Al is the world’s most productive non-ferrous metal and can be processed easily. In addition, Al metal anodes exhibit a high theoretical capacity of 2,980 mA h g^-1. Consequently, rechargeable AIBs have emerged as a promising energy-storage technology owing to their cost-effectiveness, superior safety, and high energy density^[68,69]. However, a shortage of suitable cathode materials hinders the development of AIBs. The use of 2D MXenes as cathode materials in AIBs has been extensively studied. Metal ions such as Li^+[70], Na^+[71], K^+[72], Mg^2+[53], and Al^3+[73] can be inserted into MXene layers, providing new opportunities for MMIBs that lack high-capacity and stable cathode materials^[74].

V₂CT_x MXene was used as a cathode, and its reaction mechanism in AIBs is shown in Figure 7A. During discharging process, the [Al₂Cl₇]^- ions dissociate into [AlCl₄]^- and Al³⁺ ions at the cathode/electrolyte interface, followed by the insertion of Al³⁺ ions into the MXene layers. For the anode, the Al metal undergoes a chemical reaction with [AlCl₄]^- ions, resulting in the formation of [Al₂Cl₇]^- ions. Although the etching process removes the Al layers in MXenes, most MXene layers remain in the state of multilayer stacking, which is unfavorable for ion transport and reaction kinetics. The use of interpolators to further delaminate MXenes and increase the layer spacing can improve the ion-storage capacity. Fewer-layer V₂CT_x (TBAOH-FL-V₂CT_x) [Figure 7B] was the formation of multiple-layer V₂CT_x (ML-V₂CT_x) [Figure 7C] after treatment with tetrabutylammonium hydroxide (TBAOH)^[75]. The XRD pattern [Figure 7D] shows that the interlayer spacing of TBAOH-FL-V₂CT_x increased by 5.73 Å compared to ML-V₂CT_x without TBAOH treatment. More uniform delamination endows the TBAOH-FL-V₂CT_x cathode with better electrochemical performance. The TBAOH-FL-V₂CT_x cathode had a discharge capacity of 300 mA h g^-1 at 0.1 A g^-1. In contrast, that of the ML-V₂CT_x cathode was approximately 100 mA h g^-1 under the same conditions.

Figure 7. (A) Schematic representation of the reaction mechanism of the AIBs with V₂CT_x MXene as the cathode. (B) Schematic representation of TBAOH intercalation to increase ML-V₂CT_x MXene layer spacing. (C) Schematic representation of ML-V₂CT_x etching process. (D) XRD patterns of ML-V₂CT_x and TBAOH-FL-V₂CT_x. This figure is quoted with permission from VahidMohammadi et al. Copyright (2017) American Chemical Society^[75]. (E) Schematic representation of preparing single-layer Nb₂CT_x MXene. (F) XRD patterns of P-Nb₂CT_x and C-Nb₂CT_x. This figure is quoted with permission from Li et al. Copyright (2022) American Chemical Society^[73].

Based on single-layer MXene obtained by TBAOH treatment [Figure 7E], Li et al. adjusted the single-layer Nb₂CT_x MXene surface terminated species and removed interlayer water molecules by calcination, enhancing the capacity and cycle life of the cathode^[73]. The large layer spacing (1.22 nm) caused by etching and delamination is conducive to ion transport in AIBs. In addition, the XRD patterns [Figure 7F] show that Nb₂CT_x MXene generated Nb₂O₅ and amorphous carbon after calcination, which likely contributed to the improved capacity and cycling performance of the cathode^[76,77]. After calcination at 200 °C, the C-Nb₂CT_x (200 °C-calcined Nb₂CT_x MXene) cathode had a capacity of 108 mA h g^-1 at 0.2 A g^-1 and maintained ~85% initial capacity after 500 cycles. However, under the same conditions, the specific discharge capacity of the P-Nb₂CT_x (pristine single-layered Nb₂CT_x MXene) cathode decreased to ~76 mA h g^-1 after 300 cycles.

MXenes and their derivatives as cathodes in MIBs

As an emerging advanced energy-storage technology, MIBs have garnered significant research interest, mainly because Mg is abundant (approximately 10⁴ times that of lithium), has a lower redox potential (-2.36 Vvs. SHE), and has an ultra-high volumetric capacity of up to 3,833 mA h cm^-3 (vs. 2,046 mA h cm^-3 for a lithium anode)^[78]. In addition, Mg is insensitive to moisture and air, making it more convenient for manufacturing MIBs. Mg metal, as an anode, also exhibits less dendrite growth during electrochemical deposition because of the strong bonding of Mg atoms and fast ion mobility^[79]. However, the diffusion kinetics of Mg²⁺ are considerably slower because of the strong interactions between Mg²⁺ ions and cathode materials^[10]. Consequently, MIBs exhibit low capacity and poor cycle life. Current research is focused on identifying suitable cathode materials to improve the performance of MIBs.

MXene materials have a unique layered structure that can be used directly as cathodes for Mg-ion storage in MIBs. To promote the reversible removal of Mg ions from MXenes, cetyltrimethylammonium bromide (CTAB) pre-intercalated Ti₃C₂ MXene was proposed as a cathode^[53]. Density functional theory (DFT) verified that CTA⁺ can facilitate charge transfer to MXenes, thereby enhancing the migration kinetics of Mg atoms on the MXene surface, leading to a significant change in capacity from a value close to zero to a considerable value (300 mA h cm^-3 at 50 mA g^-1, Figure 8A). In addition to CTAB, carbon nanospheres have been proven to be effective intercalated materials for enhancing the performance of MXenes. Sandwich-structured MXene@C nanospheres were prepared via electrostatic interactions between Ti₃C₂T_x and carbon nanospheres [Figure 8B]^[78]. The BET surface area results showed that Ti₃C₂T_x@C exposed more surface area (103.8 m² g^-1) than pure Ti₃C₂T_x (12.7 m² g^-1), which further verified that carbon nanospheres can alleviate the restacking of MXene. For Ti₃C₂T_x@C, the (002) peak shifted to a lower angle than that of Ti₃C₂T_x. Thus, the intercalation of carbon nanospheres can expand the layer spacing of MXene, resulting in more active sites and diffusion paths for the Mg²⁺ ions. Therefore, the electrode had excellent Mg-ion storage ability (198.7 mA h g^-1 at 0.01 A g^-1 and 123.3 mA h g^-1 at 0.2 A g^-1) [Figure 8C]. In addition to expanding the interlayer spacing, processing 2D MXene nanosheets into 3D structures is an effective strategy for enhancing the electrochemical performance of MXenes. The 3D conductive Ti₃C₂ networks were prepared by introducing bacterial celluloses (BC) into MXenes to synthesize a flexible freestanding BC/Ti₃C₂ film [Figure 8D]^[79]. XRD and DFT confirmed that the introduced BC not only increased the layer spacing but also promoted Mg-ion migration. The galvanostatic intermittent titration technique (GITT) curves revealed that BC/Ti₃C₂ had faster Mg²⁺ ion diffusion than the pure Ti₃C₂ film. Therefore, BC/Ti₃C₂ delivered a discharge capacity of 171 mA h g^-1 at 0.05 A g^-1 and 88% capacity retention after 100 cycles. Therefore, appropriate modifications are necessary to use MXenes as cathode materials for MIBs.

Figure 8. (A) GCD curves of the Ti₃C₂T_x/CTAB and Ti₃C₂T_x. This figure is quoted with permission from Xu et al. Copyright (2018) American Chemical Society^[53]. (B) Schematic illustration of the synthesis process of Ti₃C₂T_x@C. (C) Schematic illustration of the Mg//Ti₃C₂T_x@C battery. This figure is quoted with permission from Liu et al. Copyright (2019) Royal Society of Chemistry^[78]. (D) Schematic illustration of the Mg// BC/Ti₃C₂ battery. This figure is quoted with permission from Zhu et al. Copyright (2020) Elsevier^[79]. (E) Schematic illustration of the synthesis process of TiVCT_x. This figure is quoted with permission from Zhang et al. Copyright (2024) Elsevier^[80].

In addition to monometallic MXenes, bimetallic MXenes have also been investigated as energy-storage materials. Bimetallic MXenes utilize bimetallic elements at the M site and exhibit various advantages^[80]. TiVCT_x MXene not only combines the advantages of the high reducibility of V-based MXene and the high stability of Ti-based MXene, but also exhibits fewer atomic layers and higher conductivity. The synthesis process of TiVCT_x is shown in Figure 8E. Therefore, when TiVCT_x is directly used as a cathode without conductive additives, binders, and collectors^[80], the TiVCT_x film exhibits a high specific capacity (111 mA h g^-1 at 0.05 A g^-1) and excellent cyclic stability (over 1,000 cycles at 0.5 A g^-1). Ex-situ XRD, X-ray photoelectron spectroscopy (XPS), and high-resolution transmission electron microscopy (HRTEM) characterization demonstrate a reversible Mg²⁺ insertion/extraction mechanism. At present, there is little research in the field of bimetallic MXenes, and there are more bimetallic MXenes to be explored in the future. The electrochemical properties of MXenes and their derivatives as cathodes are summarized in Table 1^{[34,53,61-63,66,67,73,75,78-80]}.

Manganese oxide as cathodes

Manganese (Mn)-based materials are extensively utilized in energy-storage devices because they offer advantages such as low cost, environmental friendliness, abundant reserves, and high theoretical capacity/voltage^[81]. As a transition metal, Mn is abundant in oxidation valence states (bivalent, trivalent, and quadrivalent) and exists in a variety of Mn-based oxides, such as MnO₂, MnO, Mn₂O₃, Mn₃O₄, and ZnMn₂O₄. Among these, MnO₂ has received the most attention. The basic structural unit of MnO₂ is the MnO₆ octahedron, which can form different structures such as chains, tunnels, and layers by sharing angles/edges, resulting in different MnO₂ phases^[82]. The structures of MnO₂ materials with different crystals are shown in Figure 9. Unfortunately, Mn-based oxides used as cathodes suffer from issues such as active material dissolution, poor conductivity, and irreversible phase transitions, leading to limited cycle life and fast capacity decay^[83]. Zhao et al. reported that pure MnO₂ exhibited an initial capacity of 270 mA h g^-1 at 200 mA g^-1, which only remained at 80 mAh g^-1 after 60 cycles in ZIBs^[84]. The combination of MXenes with Mn oxides can significantly enhance the conductivity of the material, accelerate the electron transfer rates, and improve electrochemical performance. Furthermore, MXenes with hydrophilic groups can effectively boost the wettability of the cathode toward aqueous electrolytes, thereby reducing the interfacial transfer resistance.

Figure 9. The structures of MnO₂ with different crystals.

MXene/manganese oxide composites as cathodes in ZIBs

MnO₂ is the most studied Mn-based compound, and its composite with MXenes can take various forms, such as MnO₂ particles grown on the surface of MXenes and the assembly of MnO₂ nanosheets and MXene sheets by electrostatic interactions. MXenes act as a framework to ensure fast electron transfer, and the Mn dioxide provides a high capacity. Therefore, the composite material exhibits better electrochemical properties. K-V₂C@MnO₂ composites are obtained by alkalizing V₂CT_x MXenes (K-V₂C) with KOH and growing MnO₂in situ on K-V₂C using the hydrothermal method [Figure 10A]^[85]. The K-V₂C with inserted K⁺ can be used as the nucleation site of MnO₂ in hydrothermal processes; meanwhile, the insertion of K⁺ increases the layer spacing of MXenes. An SEM image of K-V₂C@MnO₂ [Figure 10B and C] shows that the formed MnO₂ grows uniformly on the K-V₂C surface. XRD patterns show that the formed MnO₂ is in the δ-MnO₂ phase (JCPDS No.80-1098). The layered V₂CT_x framework with high conductivity facilitates electron transport, effectively preventing the volume change of MnO₂, inhibiting the dissolution of MnO₂ and reducing the loss of the active material during charging and discharging. Correspondingly, the uniformly grown MnO₂ nanosheets on the V₂CT_x framework increase the specific surface area, ensuring sufficient active sites to enhance the reaction kinetics and increase the capacity. In addition, the alkalization treatment also transforms the -F group into a hydrophilic -OH group, which accelerates the wettability of the electrolyte to the cathode^[86]. Based on the above characteristics, the discharge capacity of the K-V₂C@MnO₂ cathode at 0.3 A g^-1 is 408.1 mA h g^-1. Even at 10 A g^-1, the cathode maintains a discharge capacity of 119.2 mA h g^-1 after 10,000 cycles.

Figure 10. (A) Schematic representation of the preparation of K-V₂C@MnO₂. (B and C) SEM image of K-V₂C@MnO₂. This figure is quoted with permission from Zhu et al. Copyright (2021) American Chemical Society^[85]. (D) Schematic representation of the preparation process of the MnO₂/MXene superlattice. (E) SEM image of the MnO₂/MXene. This figure is quoted with permission from Wang et al. Copyright (2023) American Chemical Society^[87]. (F) Schematic representation for the synthesis strategy of 3D Ti₃C₂T_x@MnO₂ microflowers. (G) Schematic representation of the mechanism of improved performance of 3D high-density Ti₃C₂T_x@MnO₂ microflower cathode. This figure is quoted with permission from Shi et al. Copyright (2020) Royal Society of Chemistry^[88]. (H) Schematic representation of the synthetic process for MnO₂@MXene heteronanosheets. This figure is quoted with permission from Wu et al. Copyright (2023) Elsevier^[89]. (I) The ex-situ XRD pattern of inactive ZnO byproduct generated during the cycles of ZMO cathode. (J) Schematic representation of the synthesis of ZMO@Ti₃C₂T_x composite. (K) Schematic diagram of ZMO@Ti₃C₂T_x cathode accelerating charge transfer during charging and discharging. This figure is quoted with permission from Shi et al. Copyright (2020) Elsevier^[90].

In contrast to the above-mentioned Mn dioxide particles grown on layered MXenes, a 2D MnO₂/MXene superlattice was synthesized using the solution-phase assembly method [Figure 10D]^[87]. The 2D MnO₂/MXene superlattice consisted of unilamellar MnO₂ and MXene nanosheets that stacked alternately [Figure 10E]. The high conductivity of the MXene nanosheets promoted electron transport while preventing the aggregation of MnO₂ nanosheets and enhancing their structural stability. Moreover, the regular stacking of the 2D MnO₂/MXenes exposed sufficient active sites and promoted ion transport. In addition to the common hydrothermal and electrostatic self-assembly methods, Shi et al. designed a new type of 3D microflower-like MXene-MnO₂ material combined with spray-drying technology [Figure 10F]^[88]. In this material (Ti₃C₂T_x@MnO₂), MnO₂ nanoparticles were encapsulated in pleated MXene nanosheets. The electrochemical behavior of the Ti₃C₂T_x@MnO₂ cathode in aqueous solution is shown in Figure 10G. The highly conductive Ti₃C₂T_x nanosheets provided continuous and interconnected electron transmission paths, facilitating charge transport (red arrow), and the rich functional groups of the Ti₃C₂T_x nanosheets enhanced the hydrophilicity of the electrode. In addition, the black circle in Figure 10G represents a stable skeleton that prevents structural collapse. The MnO₂ nanoparticles were wrapped by MXene nanosheets, suppressing the aggregation of MnO₂ nanoparticles and the dissolution of Mn²⁺. The 3D MXene@MnO₂ microflower cathode had a capacity of 301.2 mA h g^-1 at 0.1 A g^-1 and could undergo 2,000 cycles at 0.5 A g^-1. When the cathode reached 8.0 mg cm^-2, it still had a large capacity of 287.6 mA h g^-1 at 0.1 A g^-1.

MXenes can be used not only for aqueous ZIBs but also for non-aqueous ZIBs. The 2D δ-MnO₂@MXene heteronanosheets were obtained by deposition of δ-MnO₂ nanosheets on MXene [Figure 10H]^[89]. The deposition was achieved through a chemical reaction between KMnO₄ and polydopamine (PDA). In 0.5 M Zn triflate [Zn(OTf)₂] in triethyl phosphate (TEP) organic electrolytes, the MnO₂@MXene heteronanosheet exhibited only Zn²⁺ insertion and no H⁺ insertion, which avoided byproduct formation, Mn shuttling effects, and structural damage. At 10.5 mg cm^-2, the MnO₂@MXene heteronanosheet cathode achieved an areal capacity of 1.9 mAh cm^-2. In addition to layered Mn dioxide, ZnMn₂O₄ spinel (ZMO) is also a promising energy-storage material. However, during charging and discharging, ZMO is transformed into MnO₂. This transformation leads to the degradation of the spinel structure and the formation of irreversible inactive ZnO byproducts, resulting in poor cycling stability [Figure 10I]^[90]. Based on the above challenges, ZMO nanoparticles were assembled on Ti₃C₂T_x MXene sheets via a hydrothermal process to synthesize a ZMO@Ti₃C₂T_x composite. As shown in Figure 10J, a large number of ZMO nanoparticles were immobilized on the Ti₃C₂T_x framework, which provided a higher capacity for the cathode, whereas the Ti₃C₂T_x framework stabilized the structure and accelerated electron transport [Figure 10K]. The ZMO@Ti₃C₂T_x composite cathode exhibited a capacity of 172.6 mA h g^-1 at 0.1 A g^-1, with a capacity retention of 92.4% after 5,000 cycles at 1 A g^-1.

Vanadium oxide as cathodes

V has a variety of valence states, and V-based materials have high theoretical capacities and superior rate performances as cathodes^[91-94]. The constituent units of the V-O coordination polyhedron include tetrahedrons, triangular bipyramids, and square pyramids, and these units can change according to the oxidation state of V. Consequently, numerous V oxides can be constructed by sharing the angles and/or edges of the polyhedra. However, the main challenge in V-based materials is capacity decay owing to structural damage and the dissolution of cathodes. V₂O₅·nH₂O was reported to have an initial capacity of 273.9 mA h g^-1 and retain less than 100 mA h g^-1 after 20 cycles in ZIBs, indicating fast capacity decay^[95]. The heterogeneous interface formed by the composite of V oxide and MXene facilitated rapid electron and ion transfer, thereby enhancing the electrochemical kinetics. Furthermore, the composite exhibited remarkable energy-storage capabilities at low temperatures.

MXene/vanadium oxide composites as cathodes in ZIBs

V-based oxides are ideal candidates for ZIB cathodes due to their multielectron transfer mechanism and large layer spacing, which are favorable for ion transport^[96,97]. Designing V oxides and MXenes into heterostructures is a promising strategy for enhancing charge transport. As illustrated in Figure 11, the difference in the energy bands creates a built-in electric field within the heterostructure, facilitating electron transfer and ion diffusion. Zhang et al. prepared layer-stacked MVO + V₂C heterostructures by electrostatic self-assembly using Mn_xV₂O₆ (MVO) nanobelts and V₂C [Figure 12A], which was used as a cathode in ZIBs operating at a wide-temperature range of -20-40 °C^[98]. Many nanochannels and built-in electric fields between the interstacked Mn_xV₂O₆ and V₂CT_x achieve rapid Zn²⁺ transport and improve the intrinsic conductivity and reaction kinetics. Furthermore, the presence of numerous nanochannel interlayers enhances the wettability of the electrolyte, thereby facilitating access to the electrodes. The cycling performance of the MVO + V₂C cathode at -20 °C was tested [Figure 12B], and the results showed that the MVO + V₂C cathode had a discharge capacity of 301.3 mA h g^-1 at 0.5 A g^-1, confirming that the MXene-based heterostructure endows ZIBs with excellent performance at a wide temperature range.

Figure 11. Schematic of the built-in electric field formed by MXene and other materials.

Figure 12. (A) Schematic of the preparation process of MVO + V₂C heterostructures. (B) Cycle performance of MVO + V₂C cathode at -20 °C (0.5 A g^-1). This figure is quoted with permission from Zhang et al. Copyright (2024) John Wiley and Sons^[98]. (C) Schematic of the preparation of H₂V₃O₈ and H₂V₃O₈/MXene. This figure is quoted with permission from Liang et al. Copyright (2022) Elsevier^[100]. (D) Preparation process of HVO@Ti₃C₂ 2D heterostructure material. (E) Schematic representation of the insertion of Zn²⁺ into the HVO-Ti₃C₂ (left) and HVO@Ti₃C₂ (right) cathodes. This figure is quoted with permission from Xiao et al. Copyright (2023) John Wiley and Sons^[101]. (F) Schematic of the assembly process of V₂O₅ nanoplates and Ti₃C₂T_x MXene. (G) Schematic diagram of the dissolution of V in V₂O₅ nanoplates cathode and the inhibition of V dissolution in V₂O₅ nanoplates/MXene hybrid cathode in aqueous electrolyte. This figure is quoted with permission from Liu et al. Copyright (2022) American Chemical Society^[102]. (H) Schematic representation for the synthetic process of VOPO₄·nH₂O@MXene composite. (I) The XRD patterns of VOPO₄·nH₂O@MXene composites with different MXene content. This figure is quoted with permission from Zheng et al. Copyright (2023) John Wiley and Sons^[103].

In addition to Mn_xV₂O₆, H₂V₃O₈ nanowires have been utilized to construct a built-in electric field with MXenes to promote ion transfer. The average valence of V in H₂V₃O₈ is 4.67, and high-valence V can accommodate a large number of Zn ions^[99]. H₂V₃O₈/Ti₃C₂T_x was prepared using a one-step hydrothermal method, and the surface of delaminated Ti₃C₂T_x was uniformly covered with H₂V₃O₈ nanowires [Figure 12C]^[100]. The H₂V₃O₈ nanowires provided more active sites, whereas the MXene nanosheets increased the conductivity and ensured structural stability. The DFT calculations further revealed that the built-in electric field at the H₂V₃O₈/MXene interface induced outstanding Zn-ion diffusion kinetics and superior Zn storage capacity, which is consistent with the results shown in Figure 11. Therefore, the H₂V₃O₈/MXene electrode achieves a capacity of 437.3 mA h g^-1 at 5.0 A g^-1 and maintains structural stability over 9,000 cycles at 10 A g^-1. In addition, the 2D heterogeneous structure (2D HVO@Ti₃C₂) can also be constructed by combining V₅O₁₂·6H₂O nanosheets (HVO) with Ti₃C₂ nanosheets through electrostatic self-assembly [Figure 12D]^[101]. The researchers discovered 2D dynamic heterogeneous interface coupling; the O-Ti bonds in the interfacial V-O-Ti bonds of the 2D HVO@Ti₃C₂ cathode were reversibly broken and rebuilt during cycling. As shown in Figure 12E, during charging and discharging, owing to the built-in electric field formed by Ti₃C₂ and HVO, the heterogeneous interface enabled electrons to be reversibly transferred to the Ti₃C₂ layers through the V-O-Ti heterogeneous interface bonds, which increased the electron transfer pathway and improved the reaction kinetics. Consequently, the 2D HVO@Ti₃C₂ enhanced the capacity of the cathode (457.1 mA h g^-1 at 0.2 A g^-1).

V₂O₅, a classical V oxide, has also been combined with MXenes to form a built-in electric field. V₂O₅ nanoplates with Ti₃C₂T_x MXene layers (VPMX) were assembled using van der Waals forces [Figure 12F]^[102]. Compared to the V₂O₅ cathode, MXene coated on the surface of V₂O₅ inhibited the dissolution of V and enhanced the structural stability of the VPMX cathode during charging and discharging [Figure 12G]. As shown in Figure 11, the heterogeneous interface between V₂O₅ and MXene allowed rapid electron transfer and improved the bulk electrochemical kinetics. Additionally, the water molecules in the VPMX cathode weakened the electrostatic interactions between the cathode and Zn²⁺, accelerating ion transport and stabilizing the structure. The VPMX cathode achieved a capacity of 243.6 mA h g^-1 at 5.0 A g^-1 and a long cycling life of over 5,000 cycles at 10 A g^-1.

MXene/vanadium oxide composites as cathodes in AIBs

In addition to serving as a framework, the numerous functional groups on the MXene surface allow it to interact with other materials via the formation of M-O-Ti (M = metal atoms) and hydrogen bonds. Studies using the M-O-Ti bonds formed between VOPO₄-2H₂O and MXene to exfoliate nanosheets and obtain VOPO₄-nH₂O with different water contents have been reported. A VOPO₄·nH₂O@MXene composite was prepared by mechanical stirring to mix VOPO₄·2H₂O with different MXene contents (10%, 12%, 16%, and 48%) [Figure 12H]^[103]. The XRD patterns [Figure 12I] showed that with an increase in MXene, the (001) peak moved to a higher degree, and the number of water molecules in the VOPO₄-2H₂O interlayer was reduced, indicating that the force between MXene and VOPO₄-2H₂O affects the number of water molecules in the VOPO₄-2H₂O interlayer. The VOPO₄-H₂O@MXene composite with 12% MXene exhibited the best electrochemical properties among all MXene contents. The VOPO₄-H₂O@MXene cathode achieved a discharge capacity of 329.6 mA h g^-1 at 1 A g^-1 and a long cycling life of over 2,000 cycles at 1 A g^-1 without capacity loss. The electrochemical properties of MXene/transition metal oxide composites as cathodes are summarized in Table 2^{[85,87-90,98,100-103]}.

Sulfur-based material as cathodes

Sulfur is a lightweight, environmentally friendly element that is abundant on Earth, and sulfur cathodes have a high theoretical capacity (1,675 mA h g^-1)^[104]. Therefore, sulfur conversion cathodes offer high energy density, cost-effectiveness, and environmental friendliness and can be used in MMIBs. Except for sulfur cathodes, metal sulfides have the advantages of tunable layer spacing and high polarizability. Therefore, metal sulfides are promising candidate cathode materials for MMIBs. Sulfur atoms have a lower electronegativity than oxygen atoms, and the electrostatic interaction between the inserted multivalent ions and metal sulfides is smaller, which helps the electrode structure to remain stable. Metal sulfides, such as Mo₆S^[105], Co₉S₈^[106], MoS₂, WS₂, VS₂, and VS₄, have been used to investigate cathodes^[107]. However, sulfur cathodes face challenges such as active material polysulfide dissolution in the electrolyte and sluggish polysulfide transformation kinetics. The development of metal sulfides is hindered by their poor electrical conductivities, which are detrimental to ion transport. For example, pure MoS₂ delivers a capacity of only 62 mA h g^-1 at 0.5 A g^-1 in MIBs^[108]. The combination of MXenes with sulfur-based materials has been shown to reduce the shuttle effect of sulfur and facilitate the diffusion of metal ions. Furthermore, the large specific surface area of MXenes allows sulfides to expose more active sites in the composites, thereby enhancing the cathode capacity.

MXene/sulfur-based material composites as cathodes in ZIBs

Since the electronegativity of S^2- is lower than that of O^2-, V-based sulfide cathodes have a weaker electrostatic interaction with the bivalent Zn²⁺ than that of V-based oxide cathodes, thus allowing faster ion diffusion. As a promising cathode candidate for ZIBs, layered VS₂ has a large layer spacing and abundant V redox chemical properties. However, the relatively low electrical conductivity and severe agglomeration of VS₂ nanosheets deteriorate their electrochemical performance. A new 3D interconnected VS₂⊥V₄C₃T_x heterostructural material was prepared using the solvothermal method [Figure 13A]^[109]. VS₂ nanosheets were uniformly distributed on the surface of the V₄C₃T_x MXene framework, and this framework improved the conductivity of the cathode. Simultaneously, MXene effectively suppressed the volume change of VS₂. Thus, the use of VS₂ nanosheets as active materials contributes to their high capacity. Moreover, V₄C₃T_x MXene is used as a Zn anode coating, which inhibits the growth of Zn dendrites during cycling^[110] and provides the anode with a more uniform electric field distribution^[111]. The V₄C₃T_x@Zn symmetrical battery showed excellent cycling stability for approximately 1,700 h at 1 mA cm^-2. The full cell was assembled with the VS₂⊥V₄C₃T_x heterostructural material as the cathode and Zn foil with a protective layer of V₄C₃T_x MXene on the surface as the anode [Figure 13B]. The VS₂⊥V₄C₃T_x||V₄C₃T_x@Zn battery had a capacity of 157.1 mA h g^-1 after 5,000 cycles at 5 A g^-1.

Figure 13. (A) Schematic diagram for the preparation for the VS₂⊥V₄C₃T_x material. (B) The schematic diagram of the VS₂⊥V₄C₃T_x||V₄C₃T_x@Zn full cell. This figure is quoted with permission from Mao et al. Copyright (2023) John Wiley and Sons^[109]. (C) Schematic representation for the preparation of Co₉S₈ NP@NPC@MXene. (D) The schematic representation of the performance enhancement of the Co₉S₈ NP@NPC@MXene. This figure is quoted with permission from Yao et al. Copyright (2021) Royal Society of Chemistry^[112].

MXene/sulfur-based material composites as cathodes in AIBs

Metal-sulfide/MXene composites have also been used as cathodes for AIBs. Encapsulating the active material particles with MXenes can reduce the loss of the active material and inhibit the aggregation of nanoparticles, thereby ensuring sufficient active sites and improving the electrochemical performance of the composite cathode. The synthesis of a new type of cobalt (Co) sulfide nanoparticles/MXene composite is shown in Figure 13C^[112]. In the first step, a spongy 3D precursor of ZIF-67 encapsulated in an interlayer of MXene was prepared by in-situ growth and self-assembly. In the second step, the ZIF-67@MXene precursor and an appropriate amount of S powder were annealed at high temperatures. After carbonization and vulcanization, a 3D MXene-wrapped Co sulfide nanoparticle composite embedded in a hollow carbon nanobox (Co₉S₈ NP@NPC@MXene) was prepared. Compared to the Co₉S₈ NP@NPC composite, Co₉S₈ NP@NPC@MXene had a more stable structure. Moreover, the MXene layers in Co₉S₈ NP@NPC@MXene effectively reduced the dissolution of Co₉S₈ nanoparticles and inhibited their aggregation and overgrowth, which contributed to high capacity maintenance [Figure 13D]. The Co₉S₈ NP@NPC@MXene cathode had a capacity of 277 mA h g^-1 after 100 cycles at 0.1 A g^-1 and 110 mA h g^-1 after 1,000 cycles at 1 A g^-1.

MXene/sulfur-based material composite as cathode in MIBs

Sulfur is an ideal cathode material owing to its abundance, cost-effectiveness, and high theoretical capacity^[113]. Nevertheless, sulfur faces various challenges, such as poor electrical conductivity, slow electron and ion transport, and polysulfide shuttling^[114]. MXenes with high electronic conductivities and rich termination groups have been demonstrated to effectively increase sulfur utilization^[115]. Therefore, the combination of sulfur and MXenes can effectively exert synergistic effects on both materials. MXenes as sulfur host materials in Mg-S batteries were first reported in 2021^[116]. The S₈-sandwich cathode is composed of Ti₃C₂T_x, carbon nanotubes (CNT) and S₈ nanoparticles through low-temperature vacuum-filtration methods without a current collector or binder. Notably, as shown in Figure 14A, the S₈-sandwich is a sandwich-like architecture, where MXene acts as an interlayer with a mixed cathode of S₈ and CNT. S₈, as the source of redox-active sulfur, makes a major contribution to the capacity; CNT plays a critical role in the formation of the conductive network, and MXene with polar termination groups reduces sulfur shuttle and confines the sulfur species. The GCD curves demonstrate that the MXene interlayer significantly improves the capacity and average voltage. Therefore, MXene increases the discharge capacity of the electrode material from 290 to 530 mA h g^-1, with 83% capacity retention over 25 cycles. Furthermore, a Ti₃C₂@CoO heterostructure was synthesized as sulfur hosts via a self-assembly and annealing process^[117]. DFT confirms that CoO can guarantee more Mg polysulfide adsorption during the cycle, and Ti₃C₂ is favorable for Mg-ion diffusion [Figure 14B]. Batteries were assembled using S-Ti₃C₂@CoO as the cathode, Mg foils as the anode, and 1 M Mg(TFSI)₂/AlCl₃/diglyme as the electrolyte. The battery exhibited an excellent capacity of 540 mA h g^-1 at 100 mA g^-1 over 70 cycles. This study provides insight into MXene-based materials as sulfur host materials.

Figure 14. (A) Schematic illustration of S₈-mixed cathode. This figure is quoted with permission from Kaland et al. Copyright (2021) John Wiley and Sons^[116]. (B) Schematic diagram of magnesium polysulfide redox mechanism of the S-Ti₃C₂@CoO electrode. This figure is quoted with permission from Xu et al. Copyright (2022) Elsevier^[117]. (C) Cycle performance and SEM image of MoS₂/Ti₃C₂T_x. This figure is quoted with permission from Xu et al. Copyright (2018) Elsevier^[108]. (D) Schematic illustration of CuS/d-Ti₃C₂Tx structure. This figure is quoted with permission from Cheng et al. Copyright (2023) Royal Society of Chemistry^[118]. SEM images of (E) Ti₃C₂, (F) VS₄ and (G) VS₄@Ti₃C₂/C. This figure is quoted with permission from Zhu et al. Copyright (2022) Elsevier^[119].

The fabrication of MXene composites with metal sulfides is a common strategy for exploring new materials. A petal-like MoS₂/Ti₃C₂T_x composite was prepared using a hydrothermal method^[108]. MXene acts as a substrate to improve the conductivity of the composite, and MoS₂ grows on the MXene surface to provide abundant active sites for Mg²⁺ storage. The TEM images show that MoS₂/Ti₃C₂T_x has a fluffy petal-like nanosheet morphology and that the MoS₂ layers in the composite are thinner than those in bare MoS₂, which is beneficial for exposing more active sites and Mg-ion diffusion channels. Owing to the high conductivity and abundant surface functional groups of MXene, coupled with the high capacity of few-layer MoS₂ nanosheets, MoS₂/Ti₃C₂T_x exhibits excellent electrochemical capacity (165 mA h g^-1 at 0.05 A g^-1) [Figure 14C]. In addition to MoS₂, CuS has been shown to combine with MXene via a hydrothermal method^[118]. CuS nanocrystals are vertically distributed on delaminated Ti₃C₂T_x [Figure 14D]. The S 2p spectrum shows the two peaks at 163.66 and 164.72 eV assigned to the S-Ti-C bond, which demonstrates that CuS is anchored on the MXene surface through S-Ti-C interfacial covalent bonds, resulting in the formation of a CuS/d-Ti₃C₂T_x heterostructure. The strong coupling between CuS and d-Ti₃C₂T_x can contribute to maintaining structural stability and providing a fast charge transfer channel. DFT calculations confirmed that CuS/d-Ti₃C₂T_x exhibits a higher density of states and stronger Mg adsorption than CuS, which facilitates charge transfer and promotes reaction kinetics. The CuS/d-Ti₃C₂T_x heterostructure exhibits a capacity of 336.5 mA h g^-1 at 0.05 A g^-1 and still has a capacity of 92.2 mA h g^-1 upon 1,000 cycles at 1 A g^-1. Therefore, a few-layer metal sulfide can be combined with MXenes to achieve higher capacity and stronger stability.

In addition, V-based sulfides can be combined with MXenes via hydrothermal and electrostatic self-assembly strategies. VS₄ nanosheets have been generated in situ on a carbon-coated Ti₃C₂ MXene matrix (denoted as VS₄@Ti₃C₂/C)^[119]. According to the V 2p spectrum, two peaks were observed at 513.8 and 521.3 eV, with respect to the V-C bond. This indicates that VS₄ is anchored to the surface of Ti₃C₂. As shown in the SEM images in Figure 14E-G, owing to the MXene and VS₄ composite structural design, the morphology of VS₄ changed from a dense microsphere to a nanosheet structure; thus, VS₄ could expose more active sites and increase the contact area with the electrolyte. VS₄@Ti₃C₂/C with a unique 2D hierarchical nano-micro structure exhibited a capacity of 492 mA h g^-1 at 50 mA g^-1 and could be cycled over 900 cycles. The electrolyte in the battery was 0.25 M methylpyrrolidinium chloride + 0.25 M 2PhMgCl-AlCl₃/tetrahydrofuran. Ex-situ XRD and XPS analyses revealed the reversible insertion/extraction mechanism of MgCl⁺. Compared to Mg²⁺, MgCl⁺ exhibited a lower migration energy barrier in the cathode structure, leading to a superior electrochemical performance. In summary, when MXenes are combined with sulfides, the key to enhancing the composite capacity is to increase the number of active sites. The large specific surface area of MXenes allows sulfides to expose more active sites in the composites. The electrochemical properties of MXene/sulfur-based material composites as cathodes are summarized in Table 3^{[108,109,112,116-119]}.

Selenium-based material as cathodes

Selenium is less electronegative than oxygen and sulfur, facilitating the reduction of strong interactions between the host material and the intercalated ions. Therefore, selenium-based materials have a broad range of applications in MMIBs. Classical layered selenides have also received considerable attention. Their metal atoms covalently bond with adjacent selenium layers to form an interlayer structure (Se-M-Se), and the individual interlayer structures of Se-M-Se are connected via van der Waals forces. These properties allow rapid ion transport in the channels^[120]. However, the electrochemical stability of metal selenides is poor, mainly owing to their low electrical conductivity, volume expansion, and dissolution of the cathode active material during cycling. In addition, stacking problems may occur in layered selenides. Pure VSe₂ exhibits a discharge capacity of 159.6 mA h g^-1 at 0.5 A g^-1 in ZIBs^[121]. Therefore, the combination of selenium-based materials with MXenes having high conductivity and large ion channels can inhibit the lattice distortion caused by anion insertion during cycling and contribute to the stability of the electrode structure.

MXene/selenium-based material composites as cathodes in AIBs

MXene/selenium-based material composites provide the possibility of advanced cathodes for AIBs. The 2D composite was prepared by mixing V₂C powder with selenium (V₂C@Se) at a mass ratio of 1:2 and calcining it at a temperature of 500 °C [Figure 15A]^[122]. After selenium doping, large amounts of V²⁺ were oxidized to V⁴⁺, and functional groups such as -O, -OH, -F and -Cl on the surface of V₂C were reduced and replaced by selenium atoms^[123]. Selenium is also involved in redox reactions, and reversible transformation of Se^2- and Se²⁺ occurs during charging and discharging [Figure 15B]. Therefore, the reversible redox reactions on the V₂C@Se cathode are mainly V²⁺/V³⁺, V⁴⁺/V⁵⁺, and Se^2-/Se²⁺. The capacity of the V₂C@Se reached 402.5 mA h g^-1 at 1 A g^-1, and after 1,000 cycles, the capacity still reaches 119.8 mA h g^-1.

Figure 15. (A) Schematic of the preparation process of V₂C@Se. (B) XPS spectra of Se 3d in V₂C@Se during charging and discharging process. This figure is quoted with permission from Lv et al. Copyright (2022) Elsevier^[122]. (C) Theoretical simulation analysis with AlCl^-₄, Cl^-, and Al³⁺ entering CoSe₂. This figure is quoted with permission from Yuan et al. Copyright (2023) John Wiley and Sons^[124]. (D) Schematic illustration of the Ti₃C₂/CoSe₂ synthesis and electrochemical performance. This figure is quoted with permission from Liu et al. Copyright (2021) Elsevier^[125]. (E) Schematic illustration of the NCSe@TiVC heterostructure. This figure is quoted with permission from Zhang et al. Copyright (2022) John Wiley and Sons^[126].

To explore the mechanism of poor cycling stability and low Coulombic efficiency in AIBs, Yuan et al. analyzed the effect of embedding common carriers (AlCl^-₄, Al₂Cl^-₇, Al³⁺, and Cl^-) in CoSe₂ lattices via DFT^[124]. As shown in Figure 15C, the insertion of AlCl^-₄ into the CoSe₂ material required a large amount of energy (3.30 eV). In contrast, the energies required for the insertion of Al³⁺ and Cl^- into the CoSe₂ material were -3.27 and 0.99 eV, respectively, which did not cause large lattice distortions. Therefore, the growth of CoSe₂ on the surface of MXene can enhance the performance of cathode materials by suppressing lattice distortion. The CoSe₂@TiO₂/Ti₃C₂ cathode provided a stable discharge capacity of 197 mA h g^-1 at 0.2 A g^-1, and a retaining capacity of 102 mA h g^-1 after 500 cycles at 1.6 A g^-1 (the capacity of CoSe₂ cathode was only 25 mA h g^-1).

MXene/selenium-based material composites as cathodes in MIBs

Utilizing metal-organic frames (MOFs) to induce metal selenides is an effective strategy for preparing cathode materials. The Ti₃C₂/CoSe₂ heterostructure was prepared by the in-situ selenization of the Ti₃C₂/ZIF-67 composite [Figure 15D]^[125]. The O 1s spectrum revealed a peak at 529.1 eV, which was assigned to Co-O-Ti, suggesting a strong covalent bond between the Co atoms in CoSe₂ and the oxygen-containing functional groups from Ti₃C₂. In the heterostructure Ti₃C₂/CoSe₂, CoSe₂ provides a high capacity for Mg-ion storage, and MXene acts as a structural skeleton and promotes rapid charge transfer at the interface. Therefore, electrodes exhibit an excellent rate performance (75.7 mA h g^-1 at 1,000 m A g^-1) and cycling stability (79% capacity retention after 500 cycles). Significantly, Ti₃C₂/CoSe₂ matched with gel polymer electrolytes exhibits outstanding electrochemical performance (67.3 mA h g^-1 at 50 mA g^-1 over 70 cycles) in quasi-solid-state MIBs.

MOFs, as self-sacrificing templates, were reported to combine with TiVCT_x MXene, resulting in the formation of a NiSe₂-CoSe₂@TiVCT_x (NCSe@TiVC) heterostructure [Figure 15E]^[126]. The SEM images showed a unique 3D porous spherical structure with many active sites. DFT calculations and electrochemical impedance spectroscopy (EIS) proved that the conductivity of NCSe@TiVC was significantly improved compared with that of NiSe₂-CoSe₂. In addition, DFT was used to analyze the optimal Mg-ion adsorption sites in the structure and demonstrate that the as-prepared heterostructure promoted rapid charge transfer. NCSe@TiVC exhibited excellent electrochemical properties (136 mA h g^-1 at 0.05 A g^-1 after 100 cycles). It is worth noting that the assembled flexible pouch-cell device demonstrated excellent electrical performance (104 mA h g^-1 at 0.1 A g^-1 upon 200 cycles) when subjected to bending at various angles. The electrochemical properties of MXene/selenium-based material composites as cathodes are summarized in Table 4^{[122,124-126]}.

Other MXene composites as cathodes in ZIBs

LDHs are special layered materials consisting of multiple positively charged layers with anions in the middle, which can also be used as cathodes for ZIBs. Combining LDHs with MXene can help avoid a sharp drop in the capacity of the LDH cathode during charging and discharging. The 2D/2D Co-doped Ni-Mn-LDH/V₂CT_x MXene (CNMV) was prepared from positively charged Co-doped LDH and negatively charged 2D V₂CT_x MXene using simultaneous ion doping and electrostatic assembly methods^[127]. The LDH nanoflowers in the CNMV composite were tightly wrapped around the delaminated MXene nanosheets [Figure 16A]. Benefiting from the Co-ion doping, the layer spacing of the CNMV increased, and the CNMV cathode provided more ion transport channels and reaction sites. When the high conductivity of the 2D V₂CT_x MXene framework was combined with the high electrochemical activity of the Co-doped LDH, the energy density reached 368.7 W h kg^-1.

Figure 16. (A) Schematic representation for the synthesis of CNMV hybrid. This figure is quoted with permission from Zhang et al. Copyright (2021) Elsevier^[127]. (B) Schematic illustration for the synthesis method of TAP/Ti₃C₂T_x. (C) Electrostatic potential of TAP molecule after storing Zn²⁺ and H⁺ ions. This figure is quoted with permission from Wang et al. Copyright (2022) John Wiley and Sons^[130]. (D) SEM image of Cu-HHTP/MXene. (E) HRTEM image of Cu-HHTP/MXene. This figure is quoted with permission from Wang et al. Copyright (2022) John Wiley and Sons^[31]. (F) Schematic illustration for preparation of polyaniline/MXene hydrogel by chemical oxidation polymerization. This figure is quoted with permission from Wang et al. Copyright (2024) Royal Society of Chemistry^[134].

I₂-Zn batteries have been studied extensively owing to their low cost and safety^[128]. However, because elemental I₂ is insulating, excellent substrate conductivity is required to ensure fast electron transport. In addition, the shuttle effect of the I species (I₂, I^-, and I^-₃) during cycling causes a loss of active material, leading to capacity decay. The emergence of MXenes has provided a new approach to realize high-performance I₂-Zn batteries. The electrodeposited I₂-Nb₂CT_x MXene material (eIM) was prepared by inserting I^- ions into the MXene layers under an electric field and in-situ oxidization of I^- into linear I₂ molecules at 1.6 V^[129]. In the eIM, linear I₂ molecules were uniformly distributed on the highly conductive Nb₂CT_x MXene surface, providing more active sites and enabling fast electron transport. However, the nanoscale interlayer spacing of Nb₂CT_x MXene restricted the I species to the interlayer, reducing the loss of the active material. Therefore, the I₂-Zn battery with eIM as the cathode has an excellent rate performance (205 mA h g^-1 at 1.0 A g^-1 and 143 mA h g^-1 at 18 A g^-1).

In addition to various inorganic materials, MXenes can be combined with organic cathodes to improve the electrochemical performance. Tri (aza) pentaenene (TAP), which has extended conjugated effects along the C=N chains, was injected into Ti₃C₂T_X MXene, and the obtained TAP/Ti₃C₂T_X composite is shown in Figure 16B^[130]. Because of steric effects, the abundant C=N sites in the TAP molecules can selectively store H⁺ and Zn²⁺. Based on the electrostatic potential of the TAP molecule [Figure 16C], the C=N bonds with the lowest electrostatic potential in the blue region can store H⁺ and Zn²⁺ ions^[131,132]. The calculation results based on the charge density difference and Bader charge showed charge depletion on the surface of Ti₃C₂T_x and charge accumulation on the surface of the TAP. This demonstrated the intimate electronic interactions between TAP and Ti₃C₂T_x, which maintained the structural stability of the TAP/Ti₃C₂T_x cathode. The TAP/Ti₃C₂T_x cathode exhibited a discharge capacity of 303 mA h g^-1 at 0.04 A g^-1 and remarkable cycle stability of 10,000 times at 1 A g^-1.

MOFs are also used as advanced energy-storage materials owing to their large one-dimensional channel in MOFs^[133]. The combination with MXenes is expected to enhance the electrical conductivity and structural stability. The Cu-HHTP/MXene heterostructure material consists of alternating stacks of Cu-HHTP MOF and MXene formed by electrostatic self-assembly^[31]. The SEM [Figure 16D] and HRTEM images [Figure 16E] of Cu-HHTP/MXene showed a multilayer nanosheet stack structure. Cu-HHTP and MXene have an excellent synergistic effect; MXene nanosheets, as structural frameworks, can improve the electrical conductivity and prevent Cu-HHTP aggregation during cycling. Accordingly, the Cu-HHTP layers contribute to the capacity and act as spacer layers for the MXene nanosheets, preventing these nanosheets from stacking. The Cu-HHTP/MXene cathode has achieved a significant capacity of 260.1 mA h g^-1 at 0.1 A g^-1 and a long cycle stability over 1,000 cycles with 92.5% capacity retention at 4 A g^-1. Instead of using electrostatic adsorption to prepare heterostructures, organic molecules can be linked to MXenes through chemical reactions to form bonds. Polyaniline (PANI)/MXene hydrogels were synthesized via the rapid chemical oxidative polymerization of MXene and aniline monomers^[134]. The MXene nanosheets in the hybrid hydrogel interacted with the PANI chain through hydrogen bonding to form a 3D network structure, thereby significantly enhancing the electrochemical reactivity and stability [Figure 16F]. In addition, the 3D PANI/MXene network structure allowed rapid ion and electron transport and enhanced the reaction kinetics.

Other MXene composites as cathodes in AIBs

Organic cathodes are also an important research topic for AIBs. Although most organic cathodes are environmentally friendly and have a long cycle life, they have a low specific capacity. Combining organic cathodes with MXenes is expected to increase battery capacity. The anthraquinone derivative benzo[1,2-b:4,5-b′]dithiophene-4,8-dione (BDTO) and Ti₃C₂ MXene composite (MXene@BDTO) was synthesized using a hydrothermal method [Figure 17A]^[135]. Based on the SEM images of the MXene@BDTO and BDTO powders, BDTO was attached to the MXene layers in MXene@BDTO [Figure 17B], whereas the BDTO powders were stacked together [Figure 17C]. The MXene@BDTO structure inhibited the dissolution of BDTO and provided active reaction sites^[136]. In addition, certain Ti metal ions in MXene reacted with the C=O bonds in BDTO, resulting in the formation of C-O-Ti bonds, which enhanced the stability of structure. Therefore, the MXene@BDTO cathode exhibited an initial capacity of 229.8 mA h g^-1 at 0.5 A g^-1 and retained a discharge capacity of 134.9 mA h g^-1 even after 500 cycles. The electrochemical properties of other MXene composites as cathodes are summarized in Table 5^{[31,127,129,130,134,135]}.

Figure 17. (A) Schematic representation of the synthetic process of MXene@BDTO. (B) SEM image of MXene@BDTO. (C) SEM image of BDTO powders. This figure is quoted with permission from Wu et al. Copyright (2022) Elsevier^[135].